reviewing var
var
We use the var keyword to declare variables
When defined in a function, the var keyword scopes a variable to that function
var will hoist to the top of the scope it is defined in
You can redeclare and reassign values with var
var instructor = "Colt"; // accessible everywhere!
function greet(){
var message = "Hello!"; // scoped to the greet function
}
JavaScript
let
The let keyword creates a block-scoped variable: a variable that only exists inside a code block.
What Is A Code Block?
Essentially any pair of curly braces (outside of object syntax).
{
// this is a code block
let x = 5;
var y = 10;
}
console.log(x);
// ReferenceError: x is not defined
console.log(y);
// 10
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Where Are Code Blocks Commonly Used?
You’ll mostly use code blocks in for loops and if statements.
if (x > 10) {
let happy = true; // happy lives ONLY in this code block
}
// can't use it outside the block
console.log(happy); // ReferenceError: happy is not defined
JavaScript
An Example
for (var i = 1; i < 4; i++) {
console.log(i);
}
// 1
// 2
// 3
console.log(i);
// 4
JavaScript
for (let i = 1; i < 4; i++) {
console.log(i);
}
// 1
// 2
// 3
console.log(i); // ReferenceError: i is not defined
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More About let
It can be reassigned but not redeclared (unlike var).
let z = 5;
z = 25;
let z = 10;
// SyntaxError: Identifier 'z' has already been declared
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const
The const keyword prevents a variable from ever being reassigned or redeclared.
const PI = 3.14;
PI = 15; // TypeError: Assignment to constant variable
const PI = 5; // SyntaxError
JavaScript
const is also block-scoped, like let.
{
const x = 10;
}
console.log(x); // ReferenceError: x is not defined
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Comparison of Variable Declaration Keywords
What about var?
There’s really no need to use it
Just be careful of block scoping with let